High frequency ultrasound probe

ABSTRACT

A high frequency ultrasound probe includes a substrate having a number of transducer elements on it and a ground plane that is electrically coupled by one or more vias to a conductive frame that supports the substrate. The conductive frame is electrically coupled to a ground plane of a printed circuit having conductors that are coupled to the transducer elements.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/286,918, filed on May 23, 2014, and entitled “HIGH FREQUENCYULTRASOUND PROBE,” which claims benefit of and priority to U.S.Provisional Patent Application Ser. No. 61/827,524, filed on May 24,2013, and entitled “HIGH FREQUENCY ULTRASOUND PROBE,” both of whichapplications are hereby incorporated by reference herein in theirentireties.

TECHNICAL FIELD

The disclosed technology generally relates to the fields of ultrasonictransducers and medical diagnostic imaging. More specifically, thedisclosed technology relates to high frequency ultrasonic transducersand corresponding methods of assembly.

BACKGROUND

Ultrasonic transducers provide a means for converting electrical energyinto acoustic energy and vice versa. When the electrical energy is inthe form of an RF signal, a correctly designed transducer can produceultrasonic signals with the same frequency characteristics as thedriving electrical RF signal. Diagnostic ultrasound has traditionallybeen used at center frequencies ranging from less than 1 MHz to about 10MHz. One skilled in the art will understand that this frequency spectrumprovides a means of imaging biological tissue with resolution rangingfrom several mm to generally greater than 300 um and at depths from afew mm down to 10s of cm.

High frequency ultrasonic transducers are generally ultrasonictransducers with center frequencies above 15 MHz and ranging to over 60MHz. High frequency ultrasonic transducers provide higher resolutionwhile limiting the maximum depth of penetration, and as such, provide ameans of imaging biological tissue from a depth of a fraction of a mm toover 3 cm with resolutions in the 20 um to over 300 um range.

There are many challenges associated with fabricating high frequencyultrasonic transducers that do not arise when working with traditionalclinical ultrasonic transducers that operate at frequencies below about10 MHz. One skilled in the art will understand that structures generallyscale down according to the inverse of the frequency, so that a 50 MHztransducer will have structures about 10 times smaller than a 5 MHztransducer. In some cases, materials or techniques cannot be scaled downto the required size or shape, or in doing so they lose their functionand new technologies must be developed or adapted to allow highfrequency ultrasonic transducers to be realized. In other cases,entirely new requirements exist when dealing with the higher radiofrequency electronic and acoustic signals associated with HFUStransducers.

RF electrical interconnections require that some form of transmissionline be employed to effectively contain the magnetic fields surroundingthe signal and ground conductors. One skilled in the art will appreciatethat depending on the frequencies being transmitted, and the length ofthe conductors, electrical impedance matching and shielding techniquesmust be employed for optimal performance. One skilled in the art willfurther appreciate that at lower clinical frequencies, suchinterconnections are highly developed and available in a wide variety ofoptions to the ultrasound system and transducer designers and that suchinterconnections typically consist of several components as follows:First, a connection to the ultrasound system, which typically consistsof a zero insertion force (ZIF) type or other large format connector;second, the electrical cables running from the system connector towardthe transducer (typically micro-coaxial transmission lines); third, aninterface between the cables and the transducer usually includingconnectors and/or a printed circuit board; and finally, an interfacefrom the connector or circuit board to each of the transducer elements.This typical set of components is readily available in the industry,with many variations being successfully employed for traditionalclinical frequency US transducers.

One skilled in the art will appreciate that some of these componentswill readily scale to the higher frequencies associated with HFUS andother will not. Micro-coaxial transmission lines are well suited to thehigher frequencies associated with HFUS, and many industry standardconnector solutions are applicable at the system end as well.Furthermore, one skilled in the art will know that printed circuitboards can be designed to function at orders of magnitude higherfrequencies than those required for HFUS. The challenge for electricallyinterconnecting HFUS transducers to the ultrasound system liesprincipally in the means of making electrical connections to the actualelements of the HFUS array. These elements are very small, fragile, andoften limited to strict thermal budgets so that traditional microelectrical interconnection techniques are not suitable for HFUStransducers. Wire bonding, low temperature soldering, and ACF adhesivesfor example, are widely used technologies for making interconnections totraditional clinical frequency transducers. However, there arelimitations to these techniques that make them generally unsuitable foruse on HFUS transducers. For example, one skilled in the art willappreciate that wire bonding of interconnections at pitches less thanabout 100 um can be challenging, and at pitches below 50 um becomenearly impossible. When process temperature are limited to less thanabout 100 degrees C., wire bonding is even more challenging. Inaddition, mechanical forces associated with wire bonding becomeproblematic when substrate thickness is less than about 100 um. Typicalpiezoelectric materials suitable for making HFUS transducers must bethinned to about 100 urn down to less than 30 um for transducersspanning the 15 MHz to 50 MHz center frequency range. These thinsubstrates tend to crack when wire bonding is attempted. ACF tape andother asymmetrical conductive adhesive systems are not suitable for highreliability connections at pitches below about 200 um, and alsogenerally require a thermal budget in excess of 120 degrees Celsius,which one skilled in the art will understand, may problematic for somematerials associated with the fabrication of HFUS transducer materials.

Some HFUS transducers currently employ a grounding system that relies ona copper electrode made from thin conductive foil to be electricallyattached to the front (lens side) ground plane of the transducer, andthen exit the side of the stack and wrap around toward the flex circuitsground planes.

The primary challenge of this approach is related to the spacing of thelens to the ground plane of the piezoelectric material. In theconductive foil design, this space is equal to the thickness of thematching layers between the piezoelectric substrate and the lens, forexample, in a three matching layer device, three quarter wave matchinglayers or about 30 um at 50 MHz up to about 70 um for 20 MHz (forreference, typical printer paper is 100 um thick). This necessitates theuse of very thin foil attached to the array ground plane with a verythin bond line of conductive epoxy. Preservation of the mechanicalintegrity of the foil during subsequent lapping and adhesive/cleaningprocedures is very challenging. Other methods might be employed to allowthe use of thicker foil, but a secondary limitation of the foil is therisk of causing a delamination of the lens due to forces associated withbending the conductive foil, which are increased as the foil becomesthicker.

Finally, one skilled in the art will recognize that the conductive foiltechnique requires that the ground electrode exits the stack structurealong the edges, making electrical isolation of the device challengingespecially when BF or CF medical device ratings are required. Giventhese problems, there is a need for improved techniques of makingconnections to high frequency ultrasound transducer elements.

SUMMARY

As will be discussed and illustrated below, the disclosed technologyrelates to a high frequency ultrasound probe for transmitting andreceiving high frequency ultrasound signals and methods of assemblingsuch a probe. The probe comprises a substrate with a plurality of highfrequency transducers having a ground electrode on an outer face of thetransducers and individually connected signal electrodes on an opposite,inner face of each of the transducers. The probe also includes aplurality of vias created around the perimeter of the transducersubstrate material, which are electrically connected to the groundelectrode on the front outer side of the substrate containing thearrayed transducers. The probe further includes a support structure ofwhich at least a part is electrically conductive, and which exhibits acoefficient of thermal expansion (CTE) that is closely matched to thatof the transducer substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of theaccompanying drawings, which are incorporated in and constitute a partof this specification, and together with the description, serve toillustrate the disclosed technology.

FIG. 1 is an isometric view of the complete assembly of the arrayedtransducer labeled to illustrate the components of the grounding systemin accordance with an embodiment of the disclosed technology;

FIG. 2 is a perspective view of the top of a conductive hybrid supportstructure in accordance with an embodiment of the disclosed technology;

FIG. 3 is a perspective view of the bottom of the conductive hybridsupport structure in accordance with an embodiment of the disclosedtechnology;

FIG. 4 is a perspective view of the transducer substrate showing thepockets and metal film that will later form the vias in accordance withan embodiment of the disclosed technology;

FIG. 5 is a close up perspective view of the via pockets in accordancewith an embodiment of the disclosed technology;

FIG. 6 is a perspective view of the via pockets filled with a conductiveepoxy in accordance with an embodiment of the disclosed technology;

FIG. 7 is a perspective view looking down on the signal side of thetransducer substrate that has been thinned to the final thickness andshowing the final plated and filled vias as they are exposed from thesignal side of the substrate in accordance with an embodiment of thedisclosed technology;

FIG. 8 illustrates the assembled components of the array transducershowing the ground planes of the flex, not yet connected to theconductive support structure in accordance with an embodiment of thedisclosed technology;

FIG. 9 illustrates the fully assembled array transducer showing theground path connected by conductive adhesive between the flex circuitground plane and the conductive support structure in accordance with anembodiment of the disclosed technology;

FIG. 10 is a cross section perspective view fully labeled to illustratethe functional parts of the array transducer and the grounding system inaccordance with an embodiment of the disclosed technology;

FIG. 11 is a cross section of the grounding system showing the directpath of ground conductors from the common ground plane of the arrayedtransducers to the ground plane of the printed circuit board inaccordance with an embodiment of the disclosed technology;

FIG. 12 is a perspective view showing signal interconnections, madeusing the laser etch laser process, in place in an array transducer inaccordance with an embodiment of the disclosed technology;

FIG. 13 is a close up perspective view of the signal electrodes at oneend of the array showing the need to isolate the signal electrodes fromthe conductive portion of the support structure used to conduct theground currents to the ground plane of the flex; in accordance with anembodiment of the disclosed technology;

FIG. 14 shows a perspective view of the cross section of the array andsignal electrodes in accordance with an embodiment of the disclosedtechnology; and

FIG. 15 shows a partial cross section of the arrayed transducer withboth the signal electrodes and the ground system in place, illustratingthe insulating function of the conductive hybrid support structure.

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawing(s) will be provided by the Office upon request and paymentof the necessary fee.

DETAILED DESCRIPTION

To address the above mentioned problems, the technology disclosed hereinrelates to high frequency ultrasound transducers. As will be discussedin further detail below, one embodiment of an ultrasound probe inaccordance with the disclosed technology includes a hybrid supportstructure having an overall coefficient of thermal expansion that isclosely matched to that of a substrate material used to make an array oftransducer elements. This allows the support structure to be joined tothe transducer substrate without inducing significant stress or strainover the thermal excursions routinely seen by ultrasound transducers.

In the embodiment shown in FIG. 1, a high frequency ultrasoundtransducer 100 includes a linear (or other shaped) array of transducerelements 102, a cable bundle 104 containing a plurality of RFtransmission lines for carrying RF electrical signals between anultrasound system (not shown) and the arrayed ultrasound transducerelements, and a plurality of electrical interconnections lying betweenthe RF transmission lines and the arrayed transducer elements. Amechanical housing (not shown) encapsulates some or all of thecomponents included in the ultrasound probe assembly, such as thearrayed transducer elements, an acoustic stack and lens, and theelectrical interconnections to the cable bundle.

In one embodiment the electrical interconnections made between thetransducer elements 102 and the transmissions lines of the cable bundle104 are made according to a Laser Etch Laser (LEL) process as describedin U.S. Pat. No. 8,316,518 and U.S. patent application Ser. No.13/657,783 which are herein incorporated by reference in their entirety.

As described in the '518 patent, electrical connections are made betweena transducer element and a conductor in a flexible circuit or cablebundle by coating the conductors and transducer elements with a particle(e.g. silica) filled epoxy. The area over the transducer elements isexposed and a low fluence laser is used to create trenches in the epoxywhere conductors are desired. The number of laser pulses is increasedover portions of the flex circuits in order to dig down to the copperconductors in the flex circuits. The channeled epoxy and the transducerelements are then sputter coated with a conductor such as gold. A resistis then placed over the gold. The resist is then removed with a lowfluence laser in areas where the gold is to be removed. A wet etchprocess is then used to remove most of the gold conductor in the areaswhere it is not desired. A higher fluence laser is used to remove anyremaining gold. The resist that is located over the areas where aconductor is desired is then chemically dissolved.

FUJIFILM Sonosite, Inc. (the assignee of the present applications)developed the Laser-Etch-Laser process (referred to as LEL), which iscapable of connecting a standard flexible PCB to an ultrasonic arraystack with pitches down to less than 10 um, and within a thermal budgetof <80 degrees C. LEL-based technology is currently used in all the VSIdesigned arrays ranging from 90 um down to 38 um in pitch, and hasproven to be very reliable in the field.

In one embodiment, two flex circuits 104 are connected to either side ofthe transducer array. One circuit has conductors for the odd numberedtransducer elements while the other printed circuit has conductors forthe even numbered transducer elements. Each printed circuit hasconductors with a pitch twice that of the array.

The techniques described in the '783 application are similar except thata stepped flex circuit is used where conductors in different layers ofthe flex circuit are spaced farther apart than the distance betweenadjacent electrode elements. The conductors of the flex circuits areinterleaved so that connections can be made at a tighter pitch can bemade with having all the conductors on the same layer of a printedcircuit.

While the disclosed embodiment of the transducer probe includeselectrical interconnections made using the LEL technique, one skilled inthe art will recognize that electrical interconnections between thesignal electrodes of the array and the transmission lines of theconductor bungle could be made by alternative methods if space andthermal budgets permit.

The exemplary high frequency ultrasound probe described herein isdesigned specifically to address one of the technical challengesspecifically associated with making diagnostic images using highfrequency ultrasound transducers having a center frequency in the rangeof at least about 15 MHz and up to about 50 MHz or higher. One skilledin the art will understand that while some state of the art techniquesused for making traditional ultrasound probes, having center frequenciesbetween about 1 MHz and 10 MHz, will scale to higher frequencies, otherstechniques will not. One skilled in the art will also understand that REtransmission lines and grounding techniques must be correctly designedfor the frequencies and physical dimensions of a device to provideoptimal function, such as minimization of unwanted electricalreflections, maximizing SNR, and providing RF shielding.

In addition to the challenges associated with electrically connectingeach element to the conductor that will carry the driving signal to thetransducer, there is a need for providing a good RF ground and adequateshielding to the array. One skilled in the art will understand thatelectrical grounding of an RF device containing many elements, such asan arrayed HFUS transducer, requires specific characteristics of theground conductors to ensure low cross talk between elements, high SNRand good noise immunity, and broad band performance ensuring good signalintegrity. Ground currents must be carried away from adjacent elementsthrough low impedance conductive planes rather than wires, as inductancecan cause unpredictable behavior and ground bounce can produce unwantedcrosstalk between channels. In addition, one skilled in the art willrecognize the need to account for controlled impedance transmissionlines and impedance matching as well as EMI shielding, both of whichinfluence good RF ground design.

In the exemplary embodiment described herein, micro machined vias and aground plane are used to create a low impedance, high quality RF groundin a HF ultrasound transducer. A high frequency ultrasound probeconstructed in accordance with the disclosed technology has a highquality, low impedance RF ground plane that is both electrically andspatially efficient and mechanically and thermally robust. One skilledin the art will recognize that each transducer in the scan head cancomprise a substrate material consisting of a dielectricelectromechanical driver, such as, but not limited to, a piezoelectricceramic or a ferroelectric relaxor material, a ground electrode adjacentto one face of the driver, and a signal electrode adjacent the oppositeface, thus allowing an electric field to be imposed or measured acrossthe thickness of the electromechanical driver. One skilled in the artwill also understand that the transducer substrate may also consist of acomposite material based on a suitable electromechanical driver materialarranged in an advantageous pattern within a matrix of passivedielectric material, such as a polymer, for example an epoxy, to producean acoustic transducer having properties that are a composite of thedriver and matrix materials. One skilled in the art will furtherunderstand that this method could readily be applied to any suitabletransducer substrate consisting of a dielectric material and having abounding region for creating vias, or being bounded by a dielectric orinsulating material through which vias can be created.

In one preferred embodiment, the ground electrode is common to all thetransducer elements in the array, and is located on the distal face ofthe transducers and the substrate from which they are formed. FIGS. 4and 5 illustrate a transducer substrate 120 in which the transducerelements 102 are formed. One surface (e.g. the outer surface) of thesubstrate is covered with a conductive material 122 such as gold. Thesubstrate 120 has a number of vias 124 surrounding the perimeter of thesubstrate. The vias are plated through in order to allow the conductivematerial 122 on the outer surface of the substrate to make electricalcontact with a ground plane on the flex (or other) circuit that holdsthe transmission lines to drive the transducer elements.

The conductive material 122 forms a common ground electrode that isoverlaid by an acoustic stack typically consisting of one or morematching layers and a lens. FIG. 15 shows a cross-section of anassembled ultrasound probe including substrate 120, the vias 124, amatching layer 126 positioned over the substrate and a lens 128positioned over the matching layer 126 all of which are secured to aconductive frame 130. One skilled in the art will understand that avariety of acoustic stacks and or lens configurations are possible.

In the exemplary embodiment, the ground plane formed by the conductivematerial 122 is shown to be on the distal face of the arrayedtransducers and to be a common ground plane across the complete array oftransducers (See FIG. 4 and FIG. 10). The common planar electrodeprovides a low impedance path to ground, and provides RF shielding forthe signal electrodes located on the proximal face of the array. Theground plane located on the distal face of the transducer substrate iselectrically connected to a conductive frame 130 (FIG. 3) by theplurality of conductive vias 124 located along the perimeter of thesubstrate 120 as shown in FIGS. 4 through 7.

The conductive vias 124 are created by making pockets in the transducersubstrate 120 and then depositing an electrode material over the entireouter face of the arrayed transducers, and conformally coating the innerfaces of the via pockets 124 as shown in FIGS. 4 and 5. The pockets canbe created by any means capable of micromachining ceramics, such asreactive ion etching, laser ablation, or conventional micromachining. Inthe exemplary embodiment, laser machined vias 124 have slightly taperedwalls in the pockets to prevent shadowing during deposition of metalelectrodes on the walls of the pockets. The pockets of the vias 124 arethen filled with a conductive material, such as conductive epoxy, or anyconductive matrix that will adhere to the inner faces of the pocket (seeFIG. 6). Finally, during the processing of the arrayed transducers, thetransducer substrate 120 is thinned to the required dimension to achievethe required ultrasonic resonance in the transducers, for example, about30 um for a 50 MHz transducer.

By ensuring that the pocket is deeper than the final thickness of thetransducer substrate, the vias 124 are fully realized when thetransducer substrate is thinned to the final dimension as shown in FIG.7. By metalizing the vias before filling them with a conductivematerial, one ensures that expansion/contraction of the conductivefiller material does not result in micro cracking around the perimeterof the vias 124

The conductive frame 130 is then attached to the proximal face of thetransducer substrate using a conductive adhesive, so that the exposedconductive bottom face of the hybrid tapered support overlays theexposed upper surface of the conductive vias 124 (see FIGS. 10, 11, and15). . As can be seen in FIGS. 8, 9 and 11, the vias 124 provide aconductive path from the ground plane of the substrate 120 to theconductive frame 130. A conductive adhesive 150 is then used toelectrically couple the conductive frame to the ground plane 107 of theflexible printed circuit 104. This ensures a very low inductanceconnection between the ground plane on the flexible printed circuit andthe conductive frame 130 of the hybrid tapered support. It also ensuresthat the signal electrodes located on the inner surfaces of thetransducer substrate, the hybrid tapered support, and the PCBs arecompletely surrounded by a ground electrode providing excellentshielding.

The conductive frame 130 is part of a hybrid support structure referredto as a hybrid tapered support herein. As shown in FIG. 2, the hybridtapered support consists of a conductive outer frame 130 and aninsulating insert 140 that is secured to the outer frame byover-molding, adhesive or another technique. The insulating insert 140includes guiding pockets 142 to assist in aligning the printed flexiblecircuit to the arrayed transducers (see FIGS. 2, 3, and 13). Theconductive frame 130 is designed to have a coefficient of thermalexpansion that closely matches that of the transducer substrate 120. Inan exemplary embodiment, in the case of a ceramic substrate, such as PZTor PMN-PT, molybdenum is chosen for the conductive frame, having a CTEof about 4.8 ppm/degree C compared to 4.7 ppm/degree C for PZT.Depending on the expected thermal perturbations that the arrayedtransducers are to undergo, a closer CTE match may become more or lessimportant, allowing the choice of different materials. For example, onemight also choose engineered nickel iron alloys such as invar and itvariants designed to match the CTE of specific materials. Otherconductive materials such as graphite based materials, or platedceramics could also be suitable so long as the CTE of the frame isclosely matched to the transducer substrate over the expected thermalrange.

As best shown in FIG. 10, the ground electrode 122 is further extendedthrough the conductive support frame 130 to the ground plane 107 of aflexible printed circuit board 104. The connection to the ground planeis made over a large surface to ensure a low impedance connection to theground plane. In the exemplary embodiment, conductive adhesive is usedto connect the conductive support frame 130 to the ground plane 122 ofthe transducer substrate 120 (see FIGS. 8 through 10), although oneskilled in the art will understand that alternative methods ofconnecting the conductive frame to the ground plane could be used, suchas soldering, conductive tapes, metallization etc.

As discussed above, a plurality of signal electrodes are deposited onthe proximal face of each transducer, isolated from each other so thateach individual transducer in the array can be independently connectedto a transmitter/receiver. Each signal electrode is connected to asignal trace located on the signal layer of the flexible printed circuitboard, as a means of providing an electrical interconnection between thetransducer electrode, and the transmission line cables that carryelectrical signals to and from the ultrasound system. As describedabove, the insulating insert 140 that is secured to the conductive frame130 provides a non-conductive barrier between the transducer elements102 and the conductive frame 130.

Furthermore, in order to ensure that laser micromachining of the signalelectrodes is possible without risk exposing sections of the conductiveframe through the PCB during laser definition of the electrodes, a thinlayer of a barrier strip formed of a ceramic insulator 160 such asalumina, is bonded to the back face of the PCB 104 before bonding thePCB 104 into the pocket on the hybrid tapered support (see FIGS. 10 and11). This insulator 160 serves to stiffen and flatten the flexible PCBprior to alignment in to the tapered support making alignment of signalelectrodes in the PCB to the arrayed transducers more efficient andaccurate. In addition, the ceramic insulator prevents the creation ofshort circuits between the traces of the printed flex circuit and theconductive frame 130

One skilled in the art will understand that a conductive framesurrounding the entire perimeter of the arrayed transducer structurewill provide additional RF shielding. Furthermore, the use of aconductive ground structure with a high surface area to volume ratioensures low inductance, and when it is placed in close proximity to allof the array signal conductors ensures minimal impedance for groundcurrents and therefore better noise rejection compared to using wires toconnect the ground from the transducer to the printed circuit ground.

From the foregoing, it will be appreciated that specific embodiments ofthe invention have been described herein for purposes of illustration,but that various modifications may be made without deviating from thespirit and scope of the invention. Accordingly, the invention is notlimited except as by the appended claims.

I/We claim:
 1. A high frequency ultrasound probe comprising: aconductive frame having a coefficient of thermal expansion that isclosely matched to a transducer substrate; a substrate having a numberof transducer elements on it supported in the conductive frame; a groundplane on the substrate that is electrically coupled by one or more viasto the conductive frame.
 2. The high frequency ultrasound probe of claim1, further comprising one or more printed circuits having conductorsthat are coupled to the transducer elements and a ground plane, whereinthe ground plane of the printed circuits is electrically coupled to theconductive frame.
 3. The high frequency ultrasound probe of claim 2,wherein the ground plane of the printed circuits is electrically coupledto the conductive frame with a conductive adhesive.
 4. The highfrequency ultrasound probe of claim 2, further comprising anon-conductive barrier strip positioned between the printed circuits andthe conductive frame.
 5. The high frequency ultrasound probe of claim 1,wherein the conductive frame includes an insulating insert positionedbetween the conductive frame and the substrate.
 6. The high frequencyultrasound probe of claim 1, wherein the conductive frame and thesubstrate have similar coefficients of thermal expansion.
 7. Anultrasound transducer, comprising: a substrate having a number oftransducer elements formed therein, wherein the substrate has a commonground electrode and a number of vias filled with a conductive material;a conductive frame into which the substrate is fitted, wherein theconductive frame is electrically connected to the ground electrode onthe substrate through the vias.
 8. The ultrasound transducer of claim 7,wherein the substrate and the conductive frame have similar coefficientsof thermal expansion.
 9. The ultrasound transducer of claim 7, whereinthe vias are positioned around a perimeter of the substrate.
 10. Theultrasound transducer wherein the conductive frame has sloped wallsconfigured to support conductors to the transducer elements.